1. Field of the Invention
The invention relates to a method of inspecting the connection between a support and an alveolar structure, such as those called honeycomb structures, and to an apparatus for use in carrying out the method.
As used herein, the term honeycomb structure includes any kind of alveolar structure defining a network of parallel cells, whatever the geometry of the cells may be, and the term bonding is intended to cover any method of forming the connection between the honeycomb structure and the support, whether by glueing, welding, brazing or any other suitable means.
Because of their lightness and compression strength, the aeronautical industry is making ever-increasing use of honeycomb structures as structural reinforcement, for example, in turbomachine casings and aircraft floors. For example, it is known to place an annular band of honeycomb material on the outside of the fan casing of a turbojet engine near the fan blades as a replacement for the reinforcing ribs which were previously used and the machining of which meant that the unfinished article had to have a considerable overthickness with respect to the finished article.
This honeycomb structure is a very effective strengthening structure provided that it is bonded at the base of all of its cells to the part which is to be strengthened. To ensure that this is so, a very thorough statistical inspection of the connection must be made. If there is a fault resulting, for example, from a lack of brazing, this will manifest itself as a passage between two adjacent cells.
2. Discussion of the Prior Art
Various methods of making these inspections are known.
In a first method an operator makes an endoscopic inspection of the cell bases by brightly illuminating a cell and examining the base of the adjacent cell to see if a bonding fault allows a ray of light to pass between the illuminated cell and the adjacent cell. This check is made statistically by inspecting the honeycomb band at three or four places over its width and every five centimeters along its length. Consequently, inspecting a 15 cm wide honeycomb band bonded as a reinforcement on a fan casing of 183 cm diameter may take more than eight hours, resulting in severe visual fatigue of the operator and, therefore, a decreasing reliability of the inspection with time.
In a second method, called the capillary method, the honeycomb structure is dipped into a very wetting fluorescent liquid. The liquid is retained by capillary action in cells where the honeycomb and its bonding are free from defects, but faulty cells do not retain the liquid.
However, this method has many problems. Firstly, the liquid contains compounds, such as trichloroethane, which are dangerous to the operator since they are flammable, volatile and toxic, and hence contact with their vapors must be carefully avoided. Another disadvantage is that inspecting a casing entails dipping it into a tank to wet it, so that bulky installations are necessary. Finally, the inspection operation itself takes as long as the endoscopic inspection operation mentioned earlier.
In a third method described in EP-A-0448894 an enclosure supplied with compressed air is applied to a first zone of the honeycomb structure to be inspected, and a check is made as to whether the air can escape from the inspected zone due to bonding faults. The absence or presence of such faults can readily be confirmed by measuring the leakage discharge.
While this method is satisfactory for fairly large honeycomb structures having a cell size of at least 4 mm, it is more difficult, although not impossible, to use with smaller honeycombs. Nowadays, however, honeycomb structures having a cell size of approximately 2 mm are common. Also, this method is noisy.
Furthermore, using this method does not directly provide an accurate result. In a first phase a first measurement must be made to establish the approximate site of possible defects, after which a fine search must be carried out to locate the defects or to inspect areas which are difficult to access using a variant of the method specifically adapted to localized inspection of small areas.
This method is therefore relatively inconvenient to use.